Polypropylene resins are inexpensive and have good moldability, chemical resistance, processability, water resistance and thermal properties. Thus, they have been used as universal resins for sheets or films, car interior/exterior materials, various molded bodies, or the like.
However, since polypropylene resins have high crystallinity and low surface energy and are non-polar, they have little affinity with polar resins or coating, such as acrylic resins, urethane resins or vinyl acetate resins. As a result, there is a problem in that polypropylene resins are not amenable to coating, adhesion or printing using heterogeneous resins or coating.
There have been suggested some methods to solve such a problem. Particularly, the followings are used for carrying out coating: (1) modification of substrates: introduction of polar groups to a substrate surface using pretreatment methods such as discharge treatment, flame treatment or acid treatment, (2) use of a chlorinated resin: introducing chlorine to a polypropylene resin having a structure similar to the structure of a substrate or having high compatibility with a substrate to impart solubility and polarity, (3) use of low-molecular weight rubbery polypropylene: grafting an unsaturated carboxylic acid such as maleic acid, fumaric acid, itaconic acid or citraconic acid to a low-molecular weight soluble polypropylene resin having a structure similar to the structure of a substrate or having high compatibility with a substrate to obtain an acid-modified resin provided with polarity. In fact, use of method (1) alone shows insufficient adhesion, and thus methods (2) and (3) are frequently used in combination.
However, method (1) includes a complicated pretreatment process and causes the problems of overspend of equipment and energy and a drop in productivity. The chlorinated resin of method (2) is a high-rigidity polypropylene resin having an isotactic chain molar fraction, L, of about 1 and containing chlorine introduced thereto in order to impart polarity and solubility. Although such a chlorinated resin has been used for a long time since 1960's, there are problems in that the presence of chlorine adversely affects the users, workers and environment, deteriorates thermal stability or weather resistance of coated products, and the coated products generate environmental hormones upon discarding.
In addition, method (3) is technology free from the use of chlorine in method (2), and thus has an advantage of lack of chlorine. However, it causes additional problems in that a rubbery low-molecular weight polyolefin resin deteriorates scratch resistance and adhesion, and impurities may be attached to coated products due to the generation of tagging. Further, use of a highly crystalline resin or high-molecular weight resin for improving scratch resistance and tagging property causes degradation of coating stability and spraying property and delays in crystallization rate, resulting in surface roughening of coated bodies and generation of dust that may cause occlusion in a spraying unit. As a method for preventing tagging, an organic/inorganic nucleating agent may be used, but such a nucleating agent makes the overall process complicated, decreases transparency of the resultant coating film and causes surface precipitation.
Therefore, there has been a need in the art for a resin composition that contains no chlorine or nucleating agent, has improved scratch resistance and adhesive property, causes no tagging, and is amenable to coating using a spraying unit.